A 2 kg object accelerates as a net force acts on it. Dung the 5 seconds this for

Question

A 2 kg object accelerates as a net force acts on it. Dung the 5 seconds this force acts, the object changes velocity from 3 m/s east to 7 m/s west.

A student states:

“The initial kinetic energy of the object was 9 Joules, and the final kinetic energy was 49 Joules. Thus the change in kinetic energy is during these 5 seconds was 40 Joules, and thus the work done on this object by the net force was also 40 Joules.”

What, if anything, is wrong with this statement? If something is wrong, identify it and explain how to correct it. If this statement is correct, explain why?

Explanation / Answer

The initial kinetic energy is

KE = 1/2 mv^2 = 1/2 * 2 ( 3)^2 = 9 J

final kinetic energy

KE_f = 1/2 m v^2 = 1/2 ( 2) ( 7)^2 = - 49 J ( 0pposite direction)

the change in kinetic energy is

KE_f- KE_I = -49 J - 9 J = -58 J

W = -58 J

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